1 Field of the Invention
This invention relates to a high bandwidth information channel for carrying analog signals with electrical isolation provided by an optocoupler, and specifically to such an information channel with frequency compensation to overcome the low bandwidth characteristic of the optocoupler LED.
2. Description of the Prior Art
Prior art in the field of wideband low noise information channels includes the well-known opto-electronic isolators (optocouplers), wherein a photo-detector receives a light signal from a light emitting diode (LED). Such prior art devices have the significant disadvantage of electronic drift, i.e., performance varies with ambient humidity, temperature and time. In addition, the opto-electronic isolation technology is inherently non-linear and has limited bandwidth, so that such devices typically are able to carry signals having a maximum frequency of about 200 to 300 KHz.
It is also well known to use other techniques such as a conventional electrical transformer isolator with a V/F (voltage to frequency) converter for information channels. Such devices also have limited bandwidth and cannot carry signals with a frequency as high as 1 MHz.
Also well-known are pulse width modulators which accept an input signal and transform it into a constant frequency (i.e. A.C.) signal but with a duty factor proportional to the input signal. This constant frequency signal from the PWM is put to a conventional transformer or opto-electronic isolator, and then through a discriminator, i.e. demodulator to recover the input signal. However this device cannot carry an input signal having a frequency equal to the modulating frequency; the maximum bandwidth is about 1/3 or 1/4 of the modulating frequency, which is a significant disadvantage for carrying high frequency signals, since providing a 3 or 4 MHz modulating frequency is difficult.
The use of opto-electronic isolators is considered especially beneficial since such devices are relatively inexpensive. However, as stated above they are generally thought to provide only very limited bandwidth, i.e. have a low upper frequency limit for carrying signals. For instance, the photoconductive isolation amplifier of FIG. 1a accepts an analog input signal V.sub.in provided to the positive (non-inverting) terminal of a first operational amplifier Al, the output of which is then provided to the LED via resistor R2. The LED transmits light signals to the matched photodiode pair PD1, PD2. Photodiode PD2 provides a current output signal to output amplifier A2 which provides the output voltage signal V.sub.out. Photodetector PDI is the feedback photodiode and provides the feedback signal I.sub.PD1 back to the negative (inverting) terminal of input amplifier Ai. This feedback signal corrects for problems such as degradation of the photodetectors or degradation of the insulating medium, linearizes the LED output signal, and eliminates the time and temperature drift which is otherwise a problem with the LED. Electrical isolation is provided between the input side and output side since the only communication therebetween is light signals.
Such a circuit wherein the LED and the two photodiodes are included within a single optocoupler device (such as a Siemens IL300 optocoupler) are often considered to have a maximum bandwidth of 100 KHz, even using the frequency compensated 741-type op amp (operational amplifier) for amplifiers A1 and A2. Such prior art frequency compensated op amps "roll off" the frequency response curve well below the frequency at which the LED-induced phase lag might cause undesirable feedback oscillations. The bandwidth is therefore limited due to the inherently relatively slow response time (phase lag) of the LED.
Another similar prior art circuit shown in FIG. 1b is a photovoltaic isolation amplifier including similar components as in FIG. 1a but connected somewhat differently to provide a voltage output signal as the input signal to output amplifier A2. It is generally thought that the photoconductive photodiode operation of FIG. 1a provides the largest coupled frequency bandwidth for such optocouplers, but this is still limited to about 200 KHz maximum. Such a low bandwidth is undesirable in many applications, such as those requiring for instance an accurate direct current to 1 MHz analog information bandwidth with high input-to-output isolation. Such isolation (typically 7.5 Kilovolts) is often required for safety reasons for use in particular types of instrumentation. For instance, one application is a user interface to a switching power supply where isolation is needed between the user interface and the power supply voltage itself.
One prior art solution not using an optocoupler is described in U.S. Pat. No. 5,097,229 issued Mar. 17, 1992 to Charles 0. Forge. This discloses an analog direct current to 1 MHz information channel with high accuracy and having input/output isolation. However this patent uses a special transformer combined with a number of other components to provide the electrical isolation, and hence is relatively expensive. It would be desirable to have a circuit providing similar performances as that described in the above-referenced patent but using an opto-electronic isolator instead of a transformer.